1. Field of the Invention
The present invention concerns a nitride semiconductor device for use in photoelectronic devices in blue and ultraviolet light regions and electronic devices operating at high temperature and high power, as well as the method of manufacturing the same.
2. Description of Related Art
Nitride series semiconductors having wide band gaps such as AlN, AlGaN, GaN, GaInN and InN have been noted as materials to be applied for photoelectronic devices in blue and ultraviolet light regions and electronic devices operating under severe circumstances at high temperature and high power, and a blue light emitting diode using GaN as a main material has been attained. At present, a molecular beam epitaxy (MBE) method and an organic metal chemical vapor deposition method (MOCVD) method have been adopted mainly for growing thin films of nitride series semiconductors. The MOCVD method is adapted for transporting starting materials in a gas phase and growing them by chemical reaction on a substrate, by which an ultra thin film can be formed and mixed crystal ratio can be controlled easily by controlling the flow rate. Further, since it is possible to grow crystals at high uniformity for a large area in principle, it is an industrially important method.
However, it requires a high temperature for decomposing ammonia mainly used as a nitrogen material in the MOCVD method and it requires a substrate temperature of 900 to 1200xc2x0 C. in order to manufacture GaN crystals at high quality with a practical growing rate. Because of the high substrate temperature, it is difficult for lamination with a semiconductor which is broken at high temperature such as GaAs or GaP, which restricts the kind of substrates to be used.
Further, as the substrate for use in nitride semiconductor devices, sapphire substrates greatly different in the lattice constant have been used generally at present because bulk crystals are not available. However, due to the difference of the lattice constant, when a nitride series semiconductor is grown directly on a sapphire substrate, crystals of good quality usable for semiconductor devices can not be grown. In view of the above, a two stage growing method is used at present, in which an AlN film or a GaN film is grown at a low temperature as a buffer layer and, after elevating to a high temperature, crystals are grown subsequently. For the practical quality, it requires half width of 1.5 to 9 arcmin (0.025-0.15 degrees) in an X-ray rocking curve reflecting the fluctuation in the growing direction for crystals obtained by optimizing the buffer layer as reported by Nakamura, et al. It is considered that crystal nuclei is formed and enlarged by elevating the temperature of the buffer layer thus grown at a low temperature, thereby enabling to grow crystals during high temperature growth. Accordingly, it is difficult for the growth under the condition other than the high temperature growing condition in which crystallization proceeds effectively from amorphous or crystallite by thermal energy.
For the direct growing to a sapphire substrate, Tokuda, et al (Shingaku Giho: Technical Report of IEICE ED95-120, CPM 95-88 (1995-11) p25) reported that 13.2 arcmin (0.22 degrees) of half width for an X-ray rocking curve was obtained at a substrate temperature of 700 to 800xc2x0 C. by a plasma assisted MOCVD method, but this half width of the X-ray rocking curve is not sufficient in view of the quality for use in semiconductor devices. According to their report, the ratio between the group VA element (group No. 15, according to the revised nomenclature in the inorganic chemistry of IUPAC in 1989) and the group IIIA element (group No. 13 according to the revised nomenclature in the inorganic chemistry of IUPAC in 1989), (atom number ratio: VA group element/group IIIA element) is (35700/1) or less and the high frequency power is 150 W. Further, it has been reported that degradation occurs particularly at 700xc2x0 C. or lower under the conditions and crystals are grown as polycrystals. Accordingly, it has been difficult to directly grow a GaN film of high quality at a low temperature of 700xc2x0 C. or lower on a sapphire substrate.
On the other hand, for fabricating an electronic device, it is necessary to laminate and grow on an electrode but high substrate temperature destroys an electrode material or causes change in the state of the boundary due to thermal reaction between the electrode material and the nitride series semiconductor to deteriorate electrical characteristics as the electronic device. In view of the above, since a high substrate temperature restricts the degree of freedom for the selection of semiconductors and electrodes laminated upon fabricating a device structure, it has been demanded for a low temperature growing method.
By the way, in converting the GaN semiconductor into a p type which is essential for the device fabrication, since an optimal method at present includes a thermal annealing method in a nitrogen atmosphere and annealing is conducted at a temperature of 600xc2x0 C. or higher, low temperature growing causes a problem in the activation of Mg. In usual MOCVD method, a great amount of hydrogen and ammonia are used and Mg as a dopant for p-type conversion forms a composite compound with hydrogen and becomes inactive. In view of the problems, the plasma assisted MOCVD method can provide a low temperature growing method, as well as enables crystal growth even in a state of no substantial presence of hydrogen in the reaction system, so that this is an effective method capable of suppressing the inactivation of Mg.
However, most of nitride semiconductors manufactured by plasma assisted MOCVD method at a substrate temperature of 600xc2x0 C. or lower are amorphous, polycrystals, mixed crystals of hexagonal system and cubic system and polycrystals in which plural orientation of crystals are present together, and half width in an XRD rocking curve of a specimen fabricated at 500xc2x0 C. or lower shows a value greater by one digit or more compared with that of the high temperature MOCVD method (Materials Science Forum Vols. 264-286 (1998) 1205, Tokuda, et al, Technical Report of IEICE ED95-120, CPM95-88 (1995) 25, Jpn. J. Appl. Phys. Vol. 37 (1998) L294). Low crystallinity in the nitride semiconductor involves a problem of trapping of photo-excited or injected carriers in the semiconductor to an impurity energy level or extinction or deactivation, making it impossible for the use as light emitting/receiving devices.
For overcoming the foregoing problems in the prior art, the present invention intends to provide a nitride semiconductor device in which a nitride series compound semiconductor of high quality and excellent in crystallinity is grown directly on a substrate.
Further, the present invention intends to provide a method of manufacturing a nitride semiconductor device which can manufacture the nitride semiconductor device at a low substrate temperature.
As a result of the earnest study for overcoming the problem in the prior art, the present inventors have found that a nitride series compound semiconductor crystal can be grown directly on a substrate with no provision of a buffer layer and nitride series compound semiconductor crystals of high quality can be grown at a low temperature by making the ratio between the material of the element belonging to the group IIIA and the nitrogen material to be supplied to the substrate different from that of the prior art and by the energy possessed by superfluous excited nitrogen, and nitride series compound semiconductor crystals of high quality can be grown at a low temperature, and have accomplished the present invention.
The subject described above can be solved in accordance with the followings.
 less than 1 greater than  A nitride semiconductor device in which a nitride series compound semiconductor having at least an element belonging to the group IIIA and nitrogen is grown directly on a substrate, X-ray diffraction peaks of the nitride series compound semiconductor only include the peaks from the C-face of a hexagonal system, and the half width of an X-ray rocking curve at (0002) peak of the C-face is 0.2 degrees or less.
 less than 2 greater than  A nitride semiconductor device according to  less than 1 greater than  above, wherein a reflection high-energy electron diffraction image shows a streak image.
 less than 3 greater than  A nitride semiconductor device according to  less than 1 greater than  above, wherein the substrate has sapphire.
 less than 4 greater than  A nitride semiconductor device according to  less than 1 greater than  above, wherein the nitrogen series compound semiconductor further contains a group II element.
 less than 5 greater than  A nitride semiconductor device according to  less than 1 greater than  above, wherein the nitrogen series compound semiconductor further contains a group IV element.
 less than 6 greater than  A nitride semiconductor device according to  less than 1 greater than  above, where the device has a light emittingproperty
 less than 7 greater than  A nitride semiconductor device according to  less than 1 greater than  above, where the device has aphotoelectric conversion property.
 less than 8 greater than  A method of manufacturing a nitride semiconductor device according to  less than 1 greater than  above, which includes a step of introducing an organic metal compound at least containing a group IIIA element and a plasma activated nitrogen source into a reaction vessel to grow a nitride series compound semiconductor on the surface of a substrate, and in which the ratio for the amount of the group IIIA element and nitrogen atom supplied (ratio for the number of atoms) is group IIIA element: nitrogen atom=1:50,000 to 1:1,000,000.
 less than 9 greater than  A method of manufacturing a nitride semiconductor device according to  less than 8 greater than  above, wherein the substrate temperature is 600xc2x0 C. or lower.
 less than 10 greater than  A method of manufacturing a nitride semiconductor device according to  less than 8 greater than  above, wherein the pressure in the reaction vessel is 100 Pa or lower
 less than 11 greater than  A method of manufacturing a nitride semiconductor device according to  less than 8 greater than  above, wherein the organic metal compound containing the group IIIA element is introduced to the downstream in the flow of the plasma activated nitrogen source.  less than 12 greater than  A method of manufacturing a nitride semiconductor device according to  less than 8 greater than  above, wherein a carrier gas not containing hydrogen atoms is used.  less than 13 greater than  A method of manufacturing a nitride semiconductor device according to  less than 8 greater than , wherein a discharge power of a plasma activated unit is 300 W or higher.
For growing nitride series compound semiconductor crystals of good quality, energy is required for migration of the material substance on the surface of the substrate or removing carbon and hydrogen from the inside of crystals, in addition to decomposition of starting material gases and reaction between the group IIIA element and the nitrogen atom. In the MOCVD method, the energy required for the crystal growth is supplied by the heat of a substrate heated to a high temperature from 900xc2x0 C. to 1000xc2x0 C. or higher.
On the other hand, in the crystal growth by the existent plasma assisted MOCVD method, the nitrogen material is decomposed by plasma discharge and the group IIIA element material is decomposed by active nitrogen and also by the heat energy of the substrate. In a case where the substrate temperature is low, migration of the material element and the removal of the impurities from the surface of crystals are not sufficient only by the heat energy from the substrate. This causes growing amorphous at high concentration of impurities such as carbon or hydrogen or polycrystals with no orientation.
In the method of manufacturing a nitride semiconductor device according to the present invention, in addition to decomposition of the starting group IIIA element gas, migration on the surface of crystals and removal of impurity elements are conducted by the energy of active nitrogen activated by plasmas, providing a nitride series compound semiconductor thin film of good quality having high crystallinity and with low concentration of residual impurities even at a low substrate temperature.